Urea ((NH.sub.2).sub.2 CO) has many industrial uses. During the production of rubber and plastic articles, urea may be added as an activator of azodicarbonamide(AZO)-type blowing agents. The urea functions to lower the decomposition temperature of the AZO-type agents in sponge rubber to normal curing temperatures. It is believed that urea accomplishes this effect largely by enhancing the movement of free radicals in the compound during vulcanization. It is desirable for urea used in the production of rubber or plastic to be of a generally small particle-size, a high surface area, and to have an overall uniform particle-size distribution. Small, uniform particle-size is desirable to aid in achieving homogeneous dispersion when urea is incorporated into rubber compositions, especially in the production of foam or sponge rubber.
To obtain urea preparations for use in the manufacture of rubber, the urea, initially in the form of prills, is subjected to a mechanical grinding process. Grinding processes currently employed generally involve the use of a grinding apparatus with steel plates with teeth or pins which rotate at high speeds. This process for grinding urea produces urea preparations which are prone to agglomeration, resulting in hardened clumps and fused particles. These clumps and fused particles do not disperse sufficiently in sponge rubber compounds and ultimately contaminate the cured rubber articles.
The undesirable clumping of urea preparations is due to a number of factors. The current grinding process exposes the urea powder to atmospheric moisture and generates heat. These factors are believed to increase clumping of urea due to its hygroscopic nature. Current attempts to control moisture absorption involve adding desiccants to the urea at levels of approximately 2-4% by weight. This results, however, in an urea composition which produces more airborne particles (dust) during handling. This dust is considered a serious health hazard in rubber factories. Attempts to control dust currently involve treating the urea preparation with an oil spray (approximately 1-2% by weight). Thus, current attempts to control undesirable characteristics of urea preparations result in the addition of chemicals which reduce the purity of the urea powder by 3-6%, and impart a darker color to the urea. The coloring of the urea preparation is undesirable, as such preparations have a greater uncontrolled effect on colored rubber produced using urea.
It would be desirable to produce urea preparations of lower average particle-size and narrow particle-size distributions which do not require additives such as desiccants or dust-reducing oils. Accordingly, one object of the present invention is to provide an process for producing such urea preparations.